Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. II. Experiment

Fifth-order analogs of coherent Raman scattering generated in a number of molecular liquids using broadband quasi-cw noisy light are presented. It is seen how the signal for the direct fifth-order process, which probes the dynamics of both a fundamental vibration and its overtone, is often contaminated by a sequential process, that is only capable of probing the vibrational dynamics of the fundamental. Although these two processes are virtually indistinguishable when a single Raman resonance is excited, we find that when a second Raman resonance is available within the experimental window governed by the bandwidth of the noisy light, new frequency components in the signal arise and the two competing fifth-order processes become distinguishable. These new frequency components, as well as their decay, are explained in terms of spectral filtering of the noisy light by the Raman resonances. This spectral filter analogy predicts which of the two competing processes dominates in an equimolar mixture of benzene-$h6$ and benzene-$d6,$ and also in neat pyridine (each providing two available fundamentals). It is found by this new method that the sequential event is present and apparently fully dominates the direct process for all molecular liquids studied here.